1. Field of the Invention
The present invention relates to a method and apparatus for transmitting data from a first node to a second node of a communication system. In particular, but not exclusively, the invention relates to the transmission of data in a first and a second mode of operation between first and second nodes in a telecommunication system.
2. Description of the Related Art
As is known in the art, analogue cellular systems such as the public switched telephone network (PSTN) are referred to as first generation telecommunication systems. Digital systems such as the global system for mobile communication (GSM) have been developed and are classed as second generation systems. Third generation (3G) systems are now being designed which can support multi-media communications. The standards for this 3G system are being specified by the third generation partnership project (3GPP). These 3G mobile communication systems are often referred to as universal mobile telecommunication systems (UMTS).
In standardizing this new technology wide band code division multiple access (CDMA) (WCDMA) has emerged as the most widely accepted air interface. Indeed WCDMA has been selected as the standard for the UMTS Terrestrial Radio Access (UTRA) air interface for Frequency Division Duplexing (FDD) operation. The 3GPP has a number of technical specification groups (TSG) whose aim is to the specify whole radio access network (RAN) for the third generation mobile systems. These specifications are eventually adopted as industry standards.
One of the major steps in the evolution of these 3G systems is to provide higher capacity, data rates and media services. In order to support higher peak rates techniques such as adaptive modulation and coding (AMC) have been proposed.
AMC is the application of a particular modulation and coding scheme (MCS) depending upon the particular circumstance. Two principle schemes have been proposed. These are N phase shift keying (NPSK) which is a modulation scheme which contains only phase information, and N quadrature amplitude modulation (NQAM) in which the modulation includes phase and amplitude information. The MCS can be selected on a frame by frame basis based upon some measurement made by user equipment (UE) in the network.
In systems where only NPSK modulation is used the UE can make phase estimation from the common pilot channel (CPICH). Demodulation can be carried out for NPSK even if the amplitude level of the transport channel carrying the data is unknown.
If a higher order modulation, such as NQAM, is used both amplitude and phase information are required for demodulation. The phase information can be obtained from the CPICH but in order to calculate the amplitude information correctly the UE must know the power level for the transport channel relative to the CPICH power levels.
The problem in systems in which such high order modulation may be used is thus that the power of the transport channel cannot vary on a frame by frame (or even slot-by slot) basis without an unacceptably high quantity of signaling being used for each frame to identify that transport channel power. This signaling takes up capacity on the communication link.
It is an aim of the present invention to at least partly mitigate the above-mentioned problems.